Surface Properties of Metal Oxides

Liu, C., and Huang, P. M. (2001). The influence of catechol humification on surface properties of metal oxides. In Humic Substances Structure, Models and Functions, Ghabbour, E. A. and Davies, G, eds., Royal Society of Chemistry, Cambridge, UK, pp. 253-270. 

Before discussing the surface properties of metal oxides, it is instructive to consider some of the important bulk electronic and geometric properties that characterize them and determine their behavior. 

Parkyns ND (1969) The surface properties of metal oxides. Part II. An infrared study of the adsorption of carbon dioxide on y-alumina. J Chem Soc A4 10-417... 

The surface properties of metal oxides can be studied by a variety of methods. Different characterization methods can be used to give different information about the surface properties. No one method can be used to give a complete understanding of a surface, but integration of results gained from different techniques can lead to an understanding of the structure, reactivity, strengths, and amount of acidic and basic sites on the surface of metal oxides. 

Structure-Related Surface Properties of Metal Oxides. 

Structure-Related Surface Properties of Metal Oxides. CASE STUDIES... 

All of the knowledge that has been gained about the properties of metal oxide surfaces and interfaces, particularly over the past 8-10 years, would fill many volumes this size. There are also several dozen research groups currently... 

Formic acid is a popular molecule for probing the catalytic properties of metal oxides [23-28], The selectivity of its decomposition has frequently been used as a measure of the acid-base properties of oxides. This is a tempting generalization to make oxides that produce dehydration products (H2O and CO) are described as acidic oxides, while their basic counterparts produce dehydrogenation products (H2 + CO2). It has been shown that in many cases the product selectivity is better connected to the surface redox behavior of the oxide [29], Thus, more reducible surfaces produce higher yields of CO2, Consequently, particular attention has been paid in surface science studies to the interaction between adsorbed formate ions (the primary reaction intermediate) and surface metal cations, as well as to the participation of lattice oxygen anions in the surface reaction mechanism,... 

Like formic acid, methanol decomposition has also been used to probe the acid-base properties of metal oxides [70]. However, methoxide decomposition is dependent on surface structure in much the same way as formate decomposition. For example, methanol undergoes parallel dehydration and dehydrogenation reactions on the same crystal surface of zinc oxide [25]. Once again, product selectivity ratios may not necessarily serve as a diagnostic of acid-base properties alone. 

In most recent calorimetric studies of the acid-base properties of metal oxides or mixed metal oxides, ammonia and n-butylamine have been used as the basic molecule to characterize the surface acidity, with a few studies using pyridine, triethylamine, or another basic molecule as the probe molecule. In some studies, an acidic probe molecule like CO2 or hexafluoroisopropanol have been used to characterize the surface basicity of metal oxides. A summary of these results on different metal oxides will be presented throughout this article. Heats of adsorption of the basic gases have been frequently measured near room temperature (e.g., 35, 73-75, 77, 78,81,139-145). As demonstrated in Section 111, A the measurement of heats of adsorption of these bases at room temperature might not give accurate quantitative results owing to nonspecific adsorption. 

Electron transport properties of metal oxides nanoparticles are very important for electrical and electronic applications as well as for understanding the unique one-dimensional carrier transport mechanism. It has been noticed that the diameter of metal oxides nanoparticles, surface conditions, crystal structure and its quality i.e., chemical composition, crystallographic orientation along the film axis etc are important parameters that influence the electron transport mechanism. It is found that conductance of a nano-structure strongly depends on their crystalline structure. For example, in the case of perfect crystalline Si nanowires having four atoms per unit cell, generally three conductance channels are found [51], One-or two-atom defect, either by addition or removal of one or two atoms may disrupt the number of such conductance channel and may cause variation in the conductance. It has been observed that change in the surface conditions of the nanowires can cause remarkable... 

The phenomena presented in this book were discussed in many reviews. For example, Schwarz [13] discussed methods used to characterize the acid base properties of catalysts. The review on sorption on solid - aqueous solution interface by Parks [14] includes also principles of surface science. The book Environmental Chemistry of Aluminum edited by Sposito reviews the solution and surface chemistry of aluminum compounds. Chapter 3 [15] provides thermochemical data for aluminum compounds. Chapter 5 [16] lists the points of zero charge of aluminum oxides, oxohydroxides and hydroxides with many references on adsorption of metal cations and various anions on these materials. Unlike the present book, which is confined to sorption from solution at room temperature, publications on coprecipitation and adsorption from gas phase or at elevated temperatures are also cited there. Brown et al. [17] reviewed on dry and wet surface chemistry of metal oxides. Stumm [18] reviewed sorption of ions on iron and aluminum oxides. The review by Schindler and Stumm [19] is devoted to surface charging and specific adsorption on oxides. Schindler [19] published a review on similar topic in German. Many other reviews related to specific topics are cited in respective chapters. 

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